I live about 25 miles from Dieppe. Travel from Eurotunnel terminal, Calais to Dieppe will take about 1hour 50 minutes. From Speed Ferries at Boulogne the journey is about 10 minutes less. This is without breaking speed limits. Your total journey time will be governed by the traffic between London and your chosen port, and check-in and waiting times at UK ferry terminals. I haven't driven to London in years, but you'll know how long it takes you. Flying (via Paris) is not competitive in terms of time, but Dieppe has an aerodrome if you have a private plane.